1. Technical Field
The present invention relates to an improvement of a structure of electrodes of an active matrix type liquid crystal display (LCD) device with the so-called IPS (In Plane Switching) structure. More particularly, the present invention relates to a wiring structure of the LCD device which can remove an afterimage, and can improve a response speed and an aperture ratio by suppressing a reverse twist of liquid crystal molecules.
2. Prior Art
A display scheme has been broadly used, in which a display device using the liquid crystal material uses a liquid crystal layer of a nomadic liquid crystal in a twisted arrangement, and an electric field is applied in a vertical direction to substrates. In this scheme, two polarizing plates sandwiching the liquid crystal layer are so arranged that their polarizing axes vertically cross to each other. At the application of the electric field, the liquid crystal molecules are arranged in the vertical direction to the substrate, so that a black image is displayed. In the case that the liquid crystal molecules are arranged in the vertical direction at the application of the electric field, however, it causes a problem that the light transmitting through the liquid crystal layer in an oblique direction transmits along a birefringence path, so that the polarization direction is rotated, resulting that a full black can not be displayed to decrease a contrast ratio when an user observes the LCD device in the oblique direction, and a viewing angle for displaying the high quality image becomes narrow.
To solve such problem, the LCD device of the IPS mode in which the electric field is applied in a parallel direction to the surface of the substrates has, been proposed. It has been known that the liquid crystal molecules are rotated in a parallel plane to the surface of the substrate in the IPS mode, so that a difference between a birefringence rate at the application of the electric field and a birefringence ratio at the non-application of the electric field when the user observes the LCD device in the oblique direction becomes small, whereby the viewing angle can be broadened.
In the IPS mode for rotating the liquid crystal molecules by the electric field in the lateral direction, a method has been proposed in a Japanese Published Examined Patent Application 63-21907, which uses a pair of interdigital electrodes as the switching electrodes. In a structure proposed in Flat Panel Display, 1996, page 158, in which one switching electrode is disposed at each of a pair of edges, opposed to each other, of a light transmitting region of a square shape of one pixel element, respectively, for simplifying the structure of the electrodes to prevent a decrease of the aperture ratio, (wherein, the light transmitting region can be separated by a switching electrode into a plurality of right transmitting regions, as shown in FIGS. 3 and 4), two supplementary electrodes are disposed at each of a remaining pair of edges, opposed to each other, of the light transmitting region, respectively, and an insulating layer is sandwiched between the two supplementary capacitor electrodes to form a supplementary capacitor. This arrangement, in which the switching electrode and the supplementary capacitor electrode are integrally formed, the switching electrodes are disposed at each of one pair of edges of the light transmitting region, and the supplementary capacitor electrodes are disposed at each of the remaining pair of edges of the light transmitting region, realizes a merit for simplifying the structure of the electrodes and for improving the aperture ratio. The simplification of the electrodes is, very important in the IPS mode for the following two reasons. First, it is necessary, in the IPS mode, to make a space for the supplementary capacitor electrodes by simplifying the structure of the electrodes since the space used for the electrodes is very small, in comparison to the prior pixel element region using the twisted nomadic arrangement in which a broad transparent electrode is formed on each of the substrates, so that there is a little margin in a capacitance for storing charges. Second, since the direction of the electric field is not stabilized above the switching electrodes in the IPS mode, the image quality is degraded, so that opaque electrodes rather than the transparent electrodes are used as the material of the switching electrodes in the IPS mode, and hence if the structure of the electrodes is complicated, the aperture ratio is decreased. The merit obtained by using such structure of the electrodes is an electromagnetic shield effect of the supplementary capacitor electrodes in addition to the simplification of the structure of the electrodes. For example, in the structure shown in the FIG. 1, a bad electric effect on the orientation of the liquid crystal molecules in the light transmitting region 20 caused by a gate line 4 can be shielded by the supplementary capacitor electrode 14. Also, the bad electric effect caused by a gate line of an adjacent pixel element, not shown, can be shielded by a supplementary capacitor electrode 12. The stability of image can be improved by such electromagnetic shield effect.
In the case using such integral type supplementary capacitor electrodes, however, since the direction of the electric field at edge portions of the light transmitting region does not always vertically crosses to the switching electrodes at the application of the electric field, as described later in detail, an occurrence of the liquid crystal molecules with a reverse twist, which is opposite to the designed twist for the ideal IPS mode, can not be avoided. Even if the liquid crystal molecules are rotated in the positive twist rotative direction or the reverse twist direction, the designed correct orientation can be obtained in the case that all the liquid crystal molecules are orientated in one direction. However, if the region, in which the liquid crystal molecules are rotated in the positive twist direction, and the region, in which the liquid crystal molecules are rotated in the reverse twist direction, simultaneously exist, it takes a certain time period for the liquid crystal molecules existing in a boundary region between both the above two regions to complete the orientation, so that the problem, such as the decrease of the response speed and the display of the afterimage, is caused. Since most of such reverse twist are generated at the edge portions of the light transmitting region, it is possible to hide the afterimage in the edge portions by forming a black matrix. In this case, however, the aperture ratio is decreased.
It is an object of the present invention to decrease the occurrence of the reverse twist to improve the response speed and the aperture ratio.